Postage meters are mass produced devices for printing a defined unit value for governmental or private carrier delivery of parcels and envelopes. The term postage meter also includes other like devices which provide unit value printing such as tax stamp meters. Postage meters include internal accounting devices which account for postage value representation which is stored within the meter and is printed by the meter. As a result, postage meters must possess a high reliability to avoid the loss of user or government funds stored within the meter.
Electronic postage meters have been developed. Postage meter systems of this type are disclosed in U.S. Pat. No. 3,978,457 for MICROCOMPUTERIZED ELECTRONIC POSTAGE METER SYSTEM; and in U.S. Pat. No. 4,301,507 for ELECTRONIC POSTAGE METER HAVING PLURAL COMPUTING SYSTEM. Such meters may have electronic accounting circuits which include non-volatile memory capability for storing postage accounting information. The memory function in these electronic accounting circuits have replaced the function served in mechanical postage meters by mechanical accounting registers. The non-volatile memory and value selection in the electronic postage meters of the aforementioned patents, as well as other meter functions, may be operated under microcomputer control.
Mechanical postage meters are not subject to many of the problems encountered by electronic postage meters. Conditions cannot normally occur in mechanical postage meters, with mechanical registers, that prevent, for example, accounting for a printing cycle, improper control of the meter, or which will result in the loss of data stored in the mechanical registers. This is not the case with electronic postage meters. Critical components of electronic postage meters are subject, for example, to the effects of electromagnetic radiation and heat generated within the meter, which can effect their proper operation. Thus, precautions must be taken, as for example by proper shielding and heat reduction or dissipation, to protect the meters electronic components. Mechanical security must, of course, also be provided.
It has been recognized that it is desirable to energize electronic postage meters in a manner which eliminates or minimizes the problems associated with electromagnetic radiation. For example, a power supply system wherein only the secondary winding is within the meter secure structure is disclosed in U.S. patent application Ser. No. 344,651, filed Feb. 1, 1982, by Frank T. Check, Jr. for ELECTRONIC POSTAGE METER HAVING POWER MAGNETICALLY COUPLED TO THE METER FROM THE METER BASE. Also, the need for efficient power supplies in electronic postage meters is noted in U.S. patent application Ser. No. 306,805, filed Sept. 29, 1981, by Roland G. Miller for POWER SUPPLY SYSTEM. Further, it has been discovered that isolating high power supply system can be incorporated within the electronic postage meter itself in a manner which will provide isolation from the effects of electromagnetic radiation and the effects of both over voltage and under voltage at the input terminals to the power supply. This is achieved without having opto-coupler feedback from the secondary winding circuit to the primary winding circuit, while still retaining the necessary isolation between such windings. These discoveries are disclosed in U.S. patent application Ser. No. 506,957 filed June 23, 1983, by Arno Muller for ELECTRONIC POSTAGE METER HAVING A REGULATED POWER SUPPLY SYSTEM RESPONSIVE TO A VOLTAGE DEVELOPED IN A TRANSFORMER PRIMARY WINDING CIRCUIT. All three patent applications are assigned to Pitney Bowes Inc.
Power for the controller of many switching power supplies is often derived from the raw DC input to the power supply. However, because of the nature of regulated switching power supply circuits, many of the inputs to the controller come from the output of the supply which is the controlled variable. If it is specified, as it often is in electronic postage meters, that the output be isolated from the input, feedback becomes complicated. The situation, for such devices, becomes even more difficult if it is specified that the power supply operate over a wide range of input voltages. Moreover, the situation is further complicated when the circuit is required such as in postage meters to provide a linear feedback under a wide range of operating conditions and through years of life. Additionally, when using typical switching power supply control devices for the power supply in electronic postage meters, the circuitry may include unnecessary features such as a voltage reference, operational amplifiers, comparators, pulse width modulators, etc. thereby needlessly adding to the cost of the meter.